c++: Shrink lambda-expr

c++: Shrink lambda-expr

I noticed the default capture mode and the discriminator both used
ints.  That seems excessive.  This shrinks them to 8 bits and 16 bits
respectively.  I suppose the discriminator could use the remaining 24
bits of an int allocation unit, if we're worried about more that 64K
lambdas per function.  I know, users are strange :)  On a 64 bit system
this saves 64 bits, because we also had 32 bits of padding added.

	gcc/cp/
	* cp-tree.h (struct tree_lambda_expr): Shrink
	default_capture_mode & discriminator.